Types of Tool Marks - PowerPoint PPT Presentation

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Types of Tool Marks

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  1. Types of Tool Marks • Compression: when tool is pressed into soft material • Show outline of tool • Sliding: when tool is slides across the surface • Has a pattern of parallel striations • Class characteristics are hard to determine because many tools make similar sliding marks • Cutting: combination of compression and sliding • Quality of the tool mark is affected by receiving surface: • --Good recipients =Soft metals, plastics, and painted surfaces • --Bad recipients= Raw wood and hard metal

  2. Processing of tool marks at crime scene Identify/search for mark: First places to check: points of entry, locks, safes, etc. Do not to alter existing tool marks—no attempt to fit tool into mark Document Notes, sketch, Photographs—show location, but not useful in lab comparison Collect: Take the item back to the lab if possible A cast of the tool mark should be made if the object cannot be transported Silicone rubber is the ideal casting material

  3. Laboratory Examinations of Tool Marks • Test marks for Microscopical Comparisons • Make a test mark on a receiving surface to replicate the mark at the scene, using ductile or malleable material (to avoid damage) and the same tool • Compare the test and questioned marks • Examining Individual Characteristics • Result from manufacturing processes, wear, and damage from misuse

  4. Laboratory Examinations of Tool Marks: Conclusions that can be made • Positive ID • Class characteristics and individual characteristics match • Negative ID • Class characteristics do not match • Inconclusive • Class characteristics match but that insufficient match in individual characteristics

  5. Tool Marks on Manufactured Items Hammer marks on the heads of nails and brads Extrusion marks on pipe Machining marks on metal shavings Extrusion marks in plastic film, plastic cling wrap, and plastic bag Ream marks on sheets of flat glass Punch defect marks on illicitly manufactured drug tablets

  6. Striation Matching for Personal Identification • Human finger and toe nails have striation patterns on their upper and lower surfaces. • Size and spacing of striations is determined by dermal ridges in nail bed • Nails are prepared for examination by pressing them between two flat surfaces • Silicone rubber casts of nail ridges can be made and compared microscopically